KML is Google's Keyhole Markup Language, and is a variant of XML, used to provide coordinate information for Google Earth and similar geography applications. Converting existing data into KML is fairly straightforward, and there are a number of freely available Excel spreadsheets that will let you output KML files ready for import.
Download the XLS2KML.xls spreadsheet file (the URL is in the References section). This file uses a VBA macro and Excel's XML export functionality to pull information from specific ranges of cells and put them into appropriate KML tags.
Unzip the spreadsheet, then open it. Make sure you enable macros.
Enter the name of the item you want to see on Google Earth in the Name column. Do not leave any Name columns blank between entries, as the script will use the first blank line in the Name column as the end of range indicator.
Enter the latitude information in the appropriate column.
Enter the longitude information in the appropriate column.
Enter the description element into the appropriate column.
Enter the altitude information into the appropriate column.
Click on the button marked Create KML, and save the file to where you want it to be.
Load the KML file into Google Earth, and then save the KML file from Google Earth. (Google Earth will sort the KML elements for you, by location, and alphabetize them, saving a lot of tedious cutting and pasting in a text editor).
Download MapExcelData.xls from the URL in the References section. This file uses a VBA macro and Excel's XML export functionality to pull information from specific ranges of cells and put them into appropriate KML tags. Unzip the file and launch the spreadsheet.
Enable macros (to use the full functionality of the sheet) by selecting "Yes" on the Excel security pop-up.
Switch to the Data tab.
Enter the latitude in the Latitude column.
Enter the longitude in the Longitude column.
Enter the other information in the other columns as appropriate. Only latitude and longitude are required; fill the others as you deem necessary.
Press the button that says generate KML file. You will be prompted to save the KML file and give it a name.
Upload the KML file to Google Earth.
If you already have name, latitude and longitude information recorded in another Excel spreadsheet, you can copy and paste it into the columns of XLS2KML and save yourself a lot of typing.
The second spreadsheet mentioned is somewhat more full-featured, and can support geocaching information, and more interactivity.